Although such primary cells or "batteries" enable high energies to be obtained, because their reactive negative electrodes are corrodable in the alkaline electrolyte, they have relatively low stability which gives rise to a loss of battery capacity during storage because some of the anode is dissolved in the electrolyte. There also occurs continuous and considerable evolution of gaseous hydrogen causing electrolyte to be ejected, and sometimes giving rise to gas pressures that burst the battery's safety valve. These harmful phenomena occur more vigorously with increasing temperature, e.g. during storage in a hot country.
In order to avoid these phenomena, all battery manufacturers have for years been amalgamating their zinc electrodes, and sometimes also adding cadmium and lead thereto. Mercury and lead are simple and effective means for lowering the overvoltage at which hydrogen is given off, or in other words for avoiding corrosion of the zinc, with cadmium having the additional role of improving conduction. The zinc electrode is thus constituted by a mixture of amalgamated zinc powder containing lead and cadmium powder, and a concentrated potassium hydroxide alkaline solution, which mixture is gelled. A current collector constituted by a metal cylinder in the form of a "nail" is plunged into the electrode.
Even though these elements of mercury, cadmium, and lead are present in small quantities in each battery, there is abundant and worrying literature relating to the dangers of dispersing these substances in the environment and to the very severe risk of them finding their way into the food chain. Two things can happen to a battery after it has been thrown away by a customer. Batteries either accumulate in tips or landfills, in which case the can of the battery protects its contents for a certain length of time, after which it corrodes and the active substances including mercury are leached by surface water. Otherwise, batteries are processed in incinerators in which the mercury distills and is spread into the atmosphere with the smoke, prior to recondensing roundabout. In either event, mercury finds its way into the environment.
It is sometimes recommended that used batteries containing mercury, cadmium, or lead should be recovered, however this does not appear to be an ideal solution since it leads to a waste concentration which is even more dangerous to look after.
Therefore, looking beyond regulations which currently require the concentration of such metals in batteries to be reduced, it is essential to find ways of eliminating them completely. However, a low mercury content of less than 3% by weight relative to the zinc is already insufficient on its own for battery conservation, and various replacement solutions have already been attempted.
The most effective solutions have been proposed by the present Assignee. Thus in French patent FR-A-2 567 328, the present Assignee describes a method of stabilizing a zinc electrode by incorporating 0.01% to 1% by weight relative to the zinc of a polyfluoride organic compound of the ethoxyl fluoroalcohol type. The Assignee has also proposed, in French patent FR-A-2 583 580, another compound of the polyethoxyl alcohol and alkyl type at a concentration of 0.001% to 1% by weight. In both of these two prior cases, it has been observed that although the problem of zinc corrosion is resolved, the voltage level of the battery is reduced, and the reduction in voltage level increases with increasing current demand, i.e. the higher the current the greater the voltage drop.